The Earth's cryosphere represents a huge climate-sensitive carbon reservoir capable of releasing carbon dioxide (CO 2 ) and methane (CH 4 ) from permafrost soils or gas reservoirs capped by permafrost and ice caps upon rising global temperatures. Carbon release from these reservoirs has the potential to further accelerate global warming . Present day cryosphere demise is a focus of scientific research. The potential role of cryosphere carbon reservoirs in Mesozoic climate perturbations is even lesser known and currently underinvestigated. In contrast to previous views of a constantly warm Early Jurassic period, virtually lacking a cryosphere, recent studies have identified icehouse conditions for this time interval. Following these icehouse conditions, global warming occurred during the early Toarcian (~183 Ma) and was accompanied by a major carbon cycle anomaly as manifested in recurring negative carbon isotope excursions (CIEs). We propose that an initially volcanic-driven gentle rise of atmospheric temperature in the Early Toarcian triggered a melt-down of Earth's cryosphere which during the preceding Pliensbachian had expanded to the mid-latitudes and thus was highly vulnerable to warming. The rapid release of greenhouse gases, mainly as 13 C-depleted CH 4 , or its oxidation product CO 2 , is recorded in the carbon isotope ratios of sedimentary organic matter and carbonates. Toarcian sediments display a series of orbitally-forced negative CIEs characterized by a frequency shift from eccentricity to obliquity cycles comparable to Pleistocene climate rhythms. This pattern is explained by a self-sustaining destabilization of labile cryosphere carbon reservoirs which started at mid-latitudes where eccentricity is most effective and then rhythmically progressed poleward to latitudes where obliquity dominates. The hitherto underestimated presence of a temperature-sensitive Pliensbachian cryosphere constituted an essential precondition for the early Toarcian climate change and its associated sea-level rise. The Plie...